The Fort Gratiot Lighthouse is located at the mouth of the St. Clair River. Since it is located on a military reservation (Fort Gratiot), you'll have to go to the public park just north of the facility. There is plenty of parking and a stretch of beach on Lake Huron. Here, you'll be able to get quite close.
Map to the location is one of the additional pictures. Take Edison Parkway (riverfront) north, under the Bluewater Bridges to Garfield Ave. Right (East) to Lighthouse Park. Walk out to the beach, under the trees. The lighthouse is to your right at the entrance of the Huron River.
In the days before GPS and radio fixes, lights on the lake were the only way to know when you were nearing a dangerous reef or the entrance to a port. When the spot was too deep to build an island or a structure, lightships were anchored on the site. Crews would live on the ships taking shifts of several weeks at a time.
'Bluewater' is freshwater, whereas 'Greenwater' are the oceans. Of course 'Whitewater' is the froth created by rivers tumbling over rocks. Well that's one concept.
The Bluewater Bridge(s) spans the St. Clair River between Port Huron, Michigan, USA and Sarnia, Ontario, Canada. The older bridge, the one to the north with the heavier truss was built in the 1950's. The south span was added in the 1980's or 90's when traffic warranted more than 1 lane in each direction. Today, these spans are a monument to nearly 200 years of peace between neighbors and the peaceful trade of the world that passes underneath.